arm64: Rewrite Spectre-v4 mitigation code

Rewrite the Spectre-v4 mitigation handling code to follow the same
approach as that taken by Spectre-v2.

For now, report to KVM that the system is vulnerable (by forcing
'ssbd_state' to ARM64_SSBD_UNKNOWN), as this will be cleared up in
subsequent steps.

Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 382 insertions, 57 deletions

diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
index b373e4782ccd..b1ea935fd948 100644
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -320,79 +320,394 @@ void spectre_v2_enable_mitigation(const struct arm64_cpu_capabilities *__unused)
 	update_mitigation_state(&spectre_v2_state, state);
 }
 
-/* Spectre v4 prctl */
-static void ssbd_ssbs_enable(struct task_struct *task)
+/*
+ * Spectre v4.
+ *
+ * If you thought Spectre v2 was nasty, wait until you see this mess. A CPU is
+ * either:
+ *
+ * - Mitigated in hardware and listed in our "safe list".
+ * - Mitigated in hardware via PSTATE.SSBS.
+ * - Mitigated in software by firmware (sometimes referred to as SSBD).
+ *
+ * Wait, that doesn't sound so bad, does it? Keep reading...
+ *
+ * A major source of headaches is that the software mitigation is enabled both
+ * on a per-task basis, but can also be forced on for the kernel, necessitating
+ * both context-switch *and* entry/exit hooks. To make it even worse, some CPUs
+ * allow EL0 to toggle SSBS directly, which can end up with the prctl() state
+ * being stale when re-entering the kernel. The usual big.LITTLE caveats apply,
+ * so you can have systems that have both firmware and SSBS mitigations. This
+ * means we actually have to reject late onlining of CPUs with mitigations if
+ * all of the currently onlined CPUs are safelisted, as the mitigation tends to
+ * be opt-in for userspace. Yes, really, the cure is worse than the disease.
+ *
+ * The only good part is that if the firmware mitigation is present, then it is
+ * present for all CPUs, meaning we don't have to worry about late onlining of a
+ * vulnerable CPU if one of the boot CPUs is using the firmware mitigation.
+ *
+ * Give me a VAX-11/780 any day of the week...
+ */
+static enum mitigation_state spectre_v4_state;
+
+/* This is the per-cpu state tracking whether we need to talk to firmware */
+DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
+
+enum spectre_v4_policy {
+	SPECTRE_V4_POLICY_MITIGATION_DYNAMIC,
+	SPECTRE_V4_POLICY_MITIGATION_ENABLED,
+	SPECTRE_V4_POLICY_MITIGATION_DISABLED,
+};
+
+static enum spectre_v4_policy __read_mostly __spectre_v4_policy;
+
+static const struct spectre_v4_param {
+	const char		*str;
+	enum spectre_v4_policy	policy;
+} spectre_v4_params[] = {
+	{ "force-on",	SPECTRE_V4_POLICY_MITIGATION_ENABLED, },
+	{ "force-off",	SPECTRE_V4_POLICY_MITIGATION_DISABLED, },
+	{ "kernel",	SPECTRE_V4_POLICY_MITIGATION_DYNAMIC, },
+};
+static int __init parse_spectre_v4_param(char *str)
 {
-	u64 val = is_compat_thread(task_thread_info(task)) ?
-		  PSR_AA32_SSBS_BIT : PSR_SSBS_BIT;
+	int i;
 
-	task_pt_regs(task)->pstate |= val;
-}
+	if (!str || !str[0])
+		return -EINVAL;
 
-static void ssbd_ssbs_disable(struct task_struct *task)
-{
-	u64 val = is_compat_thread(task_thread_info(task)) ?
-		  PSR_AA32_SSBS_BIT : PSR_SSBS_BIT;
+	for (i = 0; i < ARRAY_SIZE(spectre_v4_params); i++) {
+		const struct spectre_v4_param *param = &spectre_v4_params[i];
+
+		if (strncmp(str, param->str, strlen(param->str)))
+			continue;
 
-	task_pt_regs(task)->pstate &= ~val;
+		__spectre_v4_policy = param->policy;
+		return 0;
+	}
+
+	return -EINVAL;
 }
+early_param("ssbd", parse_spectre_v4_param);
 
 /*
- * prctl interface for SSBD
+ * Because this was all written in a rush by people working in different silos,
+ * we've ended up with multiple command line options to control the same thing.
+ * Wrap these up in some helpers, which prefer disabling the mitigation if faced
+ * with contradictory parameters. The mitigation is always either "off",
+ * "dynamic" or "on".
  */
-static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
+static bool spectre_v4_mitigations_off(void)
 {
-	int state = arm64_get_ssbd_state();
+	bool ret = cpu_mitigations_off() ||
+		   __spectre_v4_policy == SPECTRE_V4_POLICY_MITIGATION_DISABLED;
 
-	/* Unsupported */
-	if (state == ARM64_SSBD_UNKNOWN)
-		return -ENODEV;
+	if (ret)
+		pr_info_once("spectre-v4 mitigation disabled by command-line option\n");
 
-	/* Treat the unaffected/mitigated state separately */
-	if (state == ARM64_SSBD_MITIGATED) {
-		switch (ctrl) {
-		case PR_SPEC_ENABLE:
-			return -EPERM;
-		case PR_SPEC_DISABLE:
-		case PR_SPEC_FORCE_DISABLE:
-			return 0;
-		}
+	return ret;
+}
+
+/* Do we need to toggle the mitigation state on entry to/exit from the kernel? */
+static bool spectre_v4_mitigations_dynamic(void)
+{
+	return !spectre_v4_mitigations_off() &&
+	       __spectre_v4_policy == SPECTRE_V4_POLICY_MITIGATION_DYNAMIC;
+}
+
+static bool spectre_v4_mitigations_on(void)
+{
+	return !spectre_v4_mitigations_off() &&
+	       __spectre_v4_policy == SPECTRE_V4_POLICY_MITIGATION_ENABLED;
+}
+
+ssize_t cpu_show_spec_store_bypass(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	switch (spectre_v4_state) {
+	case SPECTRE_UNAFFECTED:
+		return sprintf(buf, "Not affected\n");
+	case SPECTRE_MITIGATED:
+		return sprintf(buf, "Mitigation: Speculative Store Bypass disabled via prctl\n");
+	case SPECTRE_VULNERABLE:
+		fallthrough;
+	default:
+		return sprintf(buf, "Vulnerable\n");
 	}
+}
+
+enum mitigation_state arm64_get_spectre_v4_state(void)
+{
+	return spectre_v4_state;
+}
+
+static enum mitigation_state spectre_v4_get_cpu_hw_mitigation_state(void)
+{
+	static const struct midr_range spectre_v4_safe_list[] = {
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A35),
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A53),
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A55),
+		MIDR_ALL_VERSIONS(MIDR_BRAHMA_B53),
+		MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_3XX_SILVER),
+		MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_4XX_SILVER),
+		{ /* sentinel */ },
+	};
+
+	if (is_midr_in_range_list(read_cpuid_id(), spectre_v4_safe_list))
+		return SPECTRE_UNAFFECTED;
+
+	/* CPU features are detected first */
+	if (this_cpu_has_cap(ARM64_SSBS))
+		return SPECTRE_MITIGATED;
+
+	return SPECTRE_VULNERABLE;
+}
+
+static enum mitigation_state spectre_v4_get_cpu_fw_mitigation_state(void)
+{
+	int ret;
+	struct arm_smccc_res res;
+
+	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
+			     ARM_SMCCC_ARCH_WORKAROUND_2, &res);
+
+	ret = res.a0;
+	switch (ret) {
+	case SMCCC_RET_SUCCESS:
+		return SPECTRE_MITIGATED;
+	case SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED:
+		fallthrough;
+	case SMCCC_RET_NOT_REQUIRED:
+		return SPECTRE_UNAFFECTED;
+	default:
+		fallthrough;
+	case SMCCC_RET_NOT_SUPPORTED:
+		return SPECTRE_VULNERABLE;
+	}
+}
+
+bool has_spectre_v4(const struct arm64_cpu_capabilities *cap, int scope)
+{
+	enum mitigation_state state;
+
+	WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
+
+	state = spectre_v4_get_cpu_hw_mitigation_state();
+	if (state == SPECTRE_VULNERABLE)
+		state = spectre_v4_get_cpu_fw_mitigation_state();
+
+	return state != SPECTRE_UNAFFECTED;
+}
+
+static int ssbs_emulation_handler(struct pt_regs *regs, u32 instr)
+{
+	if (user_mode(regs))
+		return 1;
+
+	if (instr & BIT(PSTATE_Imm_shift))
+		regs->pstate |= PSR_SSBS_BIT;
+	else
+		regs->pstate &= ~PSR_SSBS_BIT;
+
+	arm64_skip_faulting_instruction(regs, 4);
+	return 0;
+}
+
+static struct undef_hook ssbs_emulation_hook = {
+	.instr_mask	= ~(1U << PSTATE_Imm_shift),
+	.instr_val	= 0xd500401f | PSTATE_SSBS,
+	.fn		= ssbs_emulation_handler,
+};
+
+static enum mitigation_state spectre_v4_enable_hw_mitigation(void)
+{
+	static bool undef_hook_registered = false;
+	static DEFINE_RAW_SPINLOCK(hook_lock);
+	enum mitigation_state state;
 
 	/*
-	 * Things are a bit backward here: the arm64 internal API
-	 * *enables the mitigation* when the userspace API *disables
-	 * speculation*. So much fun.
+	 * If the system is mitigated but this CPU doesn't have SSBS, then
+	 * we must be on the safelist and there's nothing more to do.
 	 */
+	state = spectre_v4_get_cpu_hw_mitigation_state();
+	if (state != SPECTRE_MITIGATED || !this_cpu_has_cap(ARM64_SSBS))
+		return state;
+
+	raw_spin_lock(&hook_lock);
+	if (!undef_hook_registered) {
+		register_undef_hook(&ssbs_emulation_hook);
+		undef_hook_registered = true;
+	}
+	raw_spin_unlock(&hook_lock);
+
+	if (spectre_v4_mitigations_off()) {
+		sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_DSSBS);
+		asm volatile(SET_PSTATE_SSBS(1));
+		return SPECTRE_VULNERABLE;
+	}
+
+	/* SCTLR_EL1.DSSBS was initialised to 0 during boot */
+	asm volatile(SET_PSTATE_SSBS(0));
+	return SPECTRE_MITIGATED;
+}
+
+/*
+ * Patch a branch over the Spectre-v4 mitigation code with a NOP so that
+ * we fallthrough and check whether firmware needs to be called on this CPU.
+ */
+void __init spectre_v4_patch_fw_mitigation_enable(struct alt_instr *alt,
+						  __le32 *origptr,
+						  __le32 *updptr, int nr_inst)
+{
+	BUG_ON(nr_inst != 1); /* Branch -> NOP */
+
+	if (spectre_v4_mitigations_off())
+		return;
+
+	if (cpus_have_final_cap(ARM64_SSBS))
+		return;
+
+	if (spectre_v4_mitigations_dynamic())
+		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
+}
+
+/*
+ * Patch a NOP in the Spectre-v4 mitigation code with an SMC/HVC instruction
+ * to call into firmware to adjust the mitigation state.
+ */
+void __init spectre_v4_patch_fw_mitigation_conduit(struct alt_instr *alt,
+						   __le32 *origptr,
+						   __le32 *updptr, int nr_inst)
+{
+	u32 insn;
+
+	BUG_ON(nr_inst != 1); /* NOP -> HVC/SMC */
+
+	switch (arm_smccc_1_1_get_conduit()) {
+	case SMCCC_CONDUIT_HVC:
+		insn = aarch64_insn_get_hvc_value();
+		break;
+	case SMCCC_CONDUIT_SMC:
+		insn = aarch64_insn_get_smc_value();
+		break;
+	default:
+		return;
+	}
+
+	*updptr = cpu_to_le32(insn);
+}
+
+static enum mitigation_state spectre_v4_enable_fw_mitigation(void)
+{
+	enum mitigation_state state;
+
+	state = spectre_v4_get_cpu_fw_mitigation_state();
+	if (state != SPECTRE_MITIGATED)
+		return state;
+
+	if (spectre_v4_mitigations_off()) {
+		arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_WORKAROUND_2, false, NULL);
+		return SPECTRE_VULNERABLE;
+	}
+
+	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_WORKAROUND_2, true, NULL);
+
+	if (spectre_v4_mitigations_dynamic())
+		__this_cpu_write(arm64_ssbd_callback_required, 1);
+
+	return SPECTRE_MITIGATED;
+}
+
+void spectre_v4_enable_mitigation(const struct arm64_cpu_capabilities *__unused)
+{
+	enum mitigation_state state;
+
+	WARN_ON(preemptible());
+
+	state = spectre_v4_enable_hw_mitigation();
+	if (state == SPECTRE_VULNERABLE)
+		state = spectre_v4_enable_fw_mitigation();
+
+	update_mitigation_state(&spectre_v4_state, state);
+}
+
+static void __update_pstate_ssbs(struct pt_regs *regs, bool state)
+{
+	u64 bit = compat_user_mode(regs) ? PSR_AA32_SSBS_BIT : PSR_SSBS_BIT;
+
+	if (state)
+		regs->pstate |= bit;
+	else
+		regs->pstate &= ~bit;
+}
+
+void spectre_v4_enable_task_mitigation(struct task_struct *tsk)
+{
+	struct pt_regs *regs = task_pt_regs(tsk);
+	bool ssbs = false, kthread = tsk->flags & PF_KTHREAD;
+
+	if (spectre_v4_mitigations_off())
+		ssbs = true;
+	else if (spectre_v4_mitigations_dynamic() && !kthread)
+		ssbs = !test_tsk_thread_flag(tsk, TIF_SSBD);
+
+	__update_pstate_ssbs(regs, ssbs);
+}
+
+/*
+ * The Spectre-v4 mitigation can be controlled via a prctl() from userspace.
+ * This is interesting because the "speculation disabled" behaviour can be
+ * configured so that it is preserved across exec(), which means that the
+ * prctl() may be necessary even when PSTATE.SSBS can be toggled directly
+ * from userspace.
+ */
+static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
+{
 	switch (ctrl) {
 	case PR_SPEC_ENABLE:
-		/* If speculation is force disabled, enable is not allowed */
-		if (state == ARM64_SSBD_FORCE_ENABLE ||
-		    task_spec_ssb_force_disable(task))
+		/* Enable speculation: disable mitigation */
+		/*
+		 * Force disabled speculation prevents it from being
+		 * re-enabled.
+		 */
+		if (task_spec_ssb_force_disable(task))
+			return -EPERM;
+
+		/*
+		 * If the mitigation is forced on, then speculation is forced
+		 * off and we again prevent it from being re-enabled.
+		 */
+		if (spectre_v4_mitigations_on())
 			return -EPERM;
+
 		task_clear_spec_ssb_disable(task);
 		clear_tsk_thread_flag(task, TIF_SSBD);
-		ssbd_ssbs_enable(task);
-		break;
-	case PR_SPEC_DISABLE:
-		if (state == ARM64_SSBD_FORCE_DISABLE)
-			return -EPERM;
-		task_set_spec_ssb_disable(task);
-		set_tsk_thread_flag(task, TIF_SSBD);
-		ssbd_ssbs_disable(task);
 		break;
 	case PR_SPEC_FORCE_DISABLE:
-		if (state == ARM64_SSBD_FORCE_DISABLE)
+		/* Force disable speculation: force enable mitigation */
+		/*
+		 * If the mitigation is forced off, then speculation is forced
+		 * on and we prevent it from being disabled.
+		 */
+		if (spectre_v4_mitigations_off())
 			return -EPERM;
-		task_set_spec_ssb_disable(task);
+
 		task_set_spec_ssb_force_disable(task);
+		fallthrough;
+	case PR_SPEC_DISABLE:
+		/* Disable speculation: enable mitigation */
+		/* Same as PR_SPEC_FORCE_DISABLE */
+		if (spectre_v4_mitigations_off())
+			return -EPERM;
+
+		task_set_spec_ssb_disable(task);
 		set_tsk_thread_flag(task, TIF_SSBD);
-		ssbd_ssbs_disable(task);
 		break;
 	default:
 		return -ERANGE;
 	}
 
+	spectre_v4_enable_task_mitigation(task);
 	return 0;
 }
 
@@ -409,22 +724,32 @@ int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
 
 static int ssbd_prctl_get(struct task_struct *task)
 {
-	switch (arm64_get_ssbd_state()) {
-	case ARM64_SSBD_UNKNOWN:
-		return -ENODEV;
-	case ARM64_SSBD_FORCE_ENABLE:
-		return PR_SPEC_DISABLE;
-	case ARM64_SSBD_KERNEL:
-		if (task_spec_ssb_force_disable(task))
-			return PR_SPEC_PRCTL | PR_SPEC_FORCE_DISABLE;
-		if (task_spec_ssb_disable(task))
-			return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
-		return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
-	case ARM64_SSBD_FORCE_DISABLE:
-		return PR_SPEC_ENABLE;
-	default:
+	switch (spectre_v4_state) {
+	case SPECTRE_UNAFFECTED:
 		return PR_SPEC_NOT_AFFECTED;
+	case SPECTRE_MITIGATED:
+		if (spectre_v4_mitigations_on())
+			return PR_SPEC_NOT_AFFECTED;
+
+		if (spectre_v4_mitigations_dynamic())
+			break;
+
+		/* Mitigations are disabled, so we're vulnerable. */
+		fallthrough;
+	case SPECTRE_VULNERABLE:
+		fallthrough;
+	default:
+		return PR_SPEC_ENABLE;
 	}
+
+	/* Check the mitigation state for this task */
+	if (task_spec_ssb_force_disable(task))
+		return PR_SPEC_PRCTL | PR_SPEC_FORCE_DISABLE;
+
+	if (task_spec_ssb_disable(task))
+		return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
+
+	return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
 }
 
 int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)